It’s a bit surprising to hear that sort of anti-originalism from Epps (more on that in a moment), but first a few thoughts in defense of Scalia (with the caveat that I'm not at all a Fourth Amendment scholar).

First, the analogy to technologies of the past isn’t as farfetched as it might seem. Justice Alito’s concurrence may have had the best line – hiding a policeman in a suspect’s coach, he said, “would have required either a gigantic coach, a very tiny constable, or both." But I’m fairly sure that in at least one of Arthur Conan Doyle’s short stories Sherlock Holmes follows a suspect by surreptitiously climbing on the back of the suspect’s carriage. And for an even better analogy, in The Sign of Four Holmes tracks a suspect who steps in creosote by using a dog to follow the scent (the stratagem goes awry when the suspect crosses paths with a wagon carrying creosote, sending the dog off on the wrong scent). True, this is fiction, and from the nineteenth century rather than the eighteenth, but surely the technological analogies aren’t unimaginable. I can think of a number of ways an 18th-century investigator might put something on a suspect’s vehicle to make it easier to identify (and, if Scalia is right about the state of the law, in general that would have been illegal absent some heightened showing). So, at least according to Scalia, there was a general rule that the government couldn’t touch your stuff in the course of an investigation. Stated in that way, the rule translates easily enough to cover placing a GPS tracker on a car.

Second, I’m not sure it’s such a bad rule for the present, at least as a constitutional minimum. Scalia says that if we don’t have some sort of fixed minimum, all of our rights may be balanced away by judges. Professor Epps seems to assume that under a modern “reasonable expectation of privacy” inquiry the defendant would still win (and maybe he would have in front of these particular Justices), but it’s far from clear that’s an obvious result. After all, the police can physically follow a suspect surreptitiously without implicating the Fourth Amendment. The GPS tracker is just less fallible and less obvious than an undercover detective, and it allows the police to monitor many more suspects. But that just means it’s more efficient, not that it’s more intrusive. How there’s a reasonable expectation of privacy in your movements on the public street is, at best, puzzling. Scalia's rule, in contrast, offers some fixed limits on the government in this context.

The better criticism is that Scalia’s rule is formalistic – it’s arbitrary to draw the line at the police’s physical touching of the suspect’s property when it’s really the police’s monitoring that we’re concerned about. As Scalia has said elsewhere, law is formalistic: it’s often an exercise in drawing a line at an arbitrary point on a spectrum on which a line must be drawn somewhere but it’s not obvious where. (Why is 55 mph perfectly safe and 56 mph too fast?). Some touchings, in the course of investigation, may be so incidental that they shouldn’t come under the Fourth Amendment, but rather than trying to figure out which those are, we include all touchings. (Whether the opposite rule holds – that police actions that don’t touch property aren’t covered – wasn’t decided in the case).

In any event, assuming Scalia is right about the 18th-century rule, his originalist point is that that’s the rule we have, until we change it in the prescribed way. Professor Epps says this is the wrong approach. But what’s his alternative? I assume it’s to have judges decide for themselves what the best modern approach should be, taking into account modern circumstances and sensibilities (which basically means doing what they think best). So really all Professor Epps is saying is that we should be ruled by modern judges, rather than by the framers’ rules. That’s the fundamental divide between originalists and advocates of a living Constitution.

If that is what Professor Epps is saying, here’s why I’m surprised to hear him say it: Not long ago he was arguing that children of illegal aliens, if born in the United States, are constitutionally entitled to U.S. citizenship by the first sentence of the Fourteenth Amendment. (I agree.) But that is a claim that we should be governed by rules laid down in the past by people who didn’t understand modern conditions. We could instead say that modern judges should be free to decide the birthright citizenship issue in light of modern circumstances (that is, however they think best). The framers of the Fourteenth Amendment didn’t know about mass illegal immigration, just as the framers of the Fourth Amendment didn’t know about GPS tracking. We can either apply the rules they wrote to new situations, or we can make up our own rules.

Whatever the answer to that dilemma, it should apply across the board. Professor Epps wants to apply the framers’ rule in one situation but make fun of Scalia for doing it another.